Hardenable alloy steels resistant to softening at elevated temperatures



w r I a an! a Patented Aug. 21, 1 951 HARDENABLE ALLOY STEEESL fRESISTANTT T SOFTENINGA'ILELEVATED IEMPER'A N. Y., a corporation of New Jersey.

No Drawing.-,.,App lication I lebruary 171950g Serial'No. 1445855 6 Claimszislcldli lzfi) This invention pertains to alloy steels ofthe'i class that are suitable for use'in' cutting' ;t oolsfa and the like, which are hardenable by quenching and which undergo secondary'fhardening on subsequent tempering, andpertains more fpa'r-' ticularly to an alloy Steel -0f this type? having a balanced composition such astoimraart-to the steel in the quenched-and tempered-condition", a highdiegree of resistancefagainst:softener; ing zwwhenz subj ected :i to 4" elevatedaistemperatures especially :within .xthe range i'extendingzaupward Fi of about 1100 F.

It is theobject of this invention'to provide a relatively low cost aalloy steelz-of 1 relatively low; total" alloy content; which, afterr 'beingiheat; treated, has better resistanceto softeningatstemperatures upwards :of about 1 1009 :thah: steels now available; t Steel of tsuchccharacteri-a isticszis useful inlpower' plant equipment-mother.

form of "bolts; studs,z'turbine zwheels; blades andibuckets; in the forging and die-casting industries for hot work dies, cores, mandrels and punches; and in the machining industry for "cut-31 ting tools.

The steel of the invention co ntains, as essential constituents, chromium, vanadium, tun a sten and molybdenum withirifibroad lnnitsfi ofi" about, 0.5 to 3% Cr, 0.2 to2%'-'V, 1 to-3%- and 0.5 to 4% Mo, togetherzwith '0.3'-to*:'1.2

carbon, a more preferred rangebeing-about,--1 1 to 2.5% Cr, 0.5 to 1.5% V, 1 to3% Wand-0.5 to 3% Mo, together withOA to"0.9%* carbon.

The balance of the compositionris substantially '2 iron, except for optional additionsiof up to about nese' mayfbe present'up to about 1%,"and silieon up to about 2%, the latter"for-"imparting" scale' zm resistance" in some analys'esa The essential constituents aforesaid are, in

accordance with the invention and for reasons explained belowQpreseht in a balanced-"composi- "Tor assuriri -'ments"-increases 2,. contentf fiii offt vaasmtgn content, 5 of the tungs'te "bont t; "a1i1"%d5*or t molybdenu'm contentsh'all totalat'least0.3% but shall not xceed sflli yt Iplus' lthe carbon content. Also forgeability bf the steel, the afore saiiiisumshouldlnot exceed" about 1.0%.

It I it well known that steels other than those which are sufiieientlyhigh in alloy content to be substantially austenitic i-at room temperature;

' loseithein strngth,.i. e.,* ?hardness,u fairly rapidly as -thertemperatui'esato which" they are-heated exceedf-about '1000" 7F. Somev :ste'els containing thei lcarbideforming elements: chromium; vana-- dium, tungsten -Ian'demolybdenum; :are much sistantrthaIr-other steels'to this weaken ing" levated temperatures;because when they are@properlyheat treated":there-occurs in then'ra precipitation" 01 1 -a1l'oy-- fcai'bides at tempera-*- tures upwards=-0f':ab'out--1000 F., and this precipitation prevents or retards the softening or weakening at f high temperatures. The heat treatment referred to involves a heating to a temperatui'e' highs nough-Ito causal-an appreciable aniou'ntbf carbonito'dis'sblve in the steel; followed by a cooling to approximately room temperature at'a sufliciently rapid rate to prevent much precipitation of the carbides 'during the cooling; and finallica ,temperingatabout 1000 to 1200 1,

It hasbeen': generally accepted that the larger th-amount-of carbide lforming elements -in--the-- steel,-:the greater; is the resistance of-the steel to softening a't temperatures'upwards of about-1000-"" F. This has beIhWell illustrated by: E. C(Biiih in ChaptenVI-ef his book Functions of the Alloy- 3 mg Elements in Steel? "Now I have discovered, and this is the" basis 'of my" invention, that the resistance to softening particularly at tempera-- ture's' "upwards 6f ab'out1200 F. may decrease in" some stee'lsas the e n-oust of the alloying "ele" This ts illustrated "in the "foblowing TablsI Q" I-inclusive. "-'Io'getthedata for these tables,- he s'amples treatedwerefirst heatedqz'o to' 'assure'thoroughsolution of carbides and" their were quenchedi 'Ihey-'-weretion sueh .that the-sum of "th hr m :tested f hardH' ss arsewmum 'ive reheatmgs at the successively higher temperatures indicated in the tables.

increase the resistance of steels to softening on heating to temperatures on the order aforesaid.

TABLE I Effect of chromium on hardness of steels after tempering Rockwell O Hardness Alter Cumulative Analysis, Per Cent 2 Hour Temp'erings Bar Cr v w Mo 1150 r. 1200* F. 1450 F. 1300" F. 1350 F.

.41 1. .56 .95 .75 43. 5 47. 5 44. 5 41 33. 5 .37 1. 97 .55 .93 .72 4s. 5 45 42.5 33. 5 32. 5 .40 3. 09 .54 .93 .71 49. 5 46 41 35. 5 29. 5 .41 2. 26 .59 .65 .46 4s. 5 46 41 36.5 30 .41 3.30 .60 .65 .45 43. 5 44 37 32. 5 2s 39 1.13 .34 1. 69 1. 53 54 52. 5 50 45. 5 39. 5 .37 2. 01 .32 1. 70 1. 43 53. 5 51. 5 47. 5 42. 5 34. 5

Y .36 2. 92 92 1. 7o 1. 59 53. 5 50. 5 45 3s. 5 31 1 Heated to 2250 F., instead of 2350 F. to avoid melting the steel.

It can readily be seen in each of the above groups, that as the chromium increases from about 1.0 to 4.0% the hardness decreases for a given reheating temperature above about 1200 F.

It, therefore, appears that some other factors evidently control the hardness of the steel after tempering, and I have discovered the principle that the resistance to softening, rather than being TABLE II Effect of vanadium on hardness of steels after tempering Analysis, Per Cent g E3$ $g E Bar 0 Cr v w Mo 1150 F. 1200 F. 1250 F. 1300 F. 1350 F.

Although the effect with vanadium is not so directly proportional to the total of the carbide marked as it is with chromium, the tendency is 40 forming elements, is dependent on certain ratios clearly indicated in Table II particularly for reof these elements and the carbon content of the heatings at 1250 and 1300 F., namely, that insteel. As stated by Crafts and Lamont in Tech. creasing the vanadium content from about 0.9 Pub. 2439, AIME, 1948, the probable formulae of to 1.4% causesadecrease in hardness. the carbides of chromium, vanadium, tungsten It was also found to be true, as shown in Table 45 and molybdenum in the steels which have sec- III, that steels containing large totals of Cr, V, W, ondary hardening, are, respectively, CI7C3; V4C3; and M0 were less resistant to softening, particu- W2C and M020. The proportions by weight of larly at temperatures over about 1200 F., than carbon in these compounds are .lOxpercent Cr steels containing smaller total amounts of these for the carbide Cr7C3; .18 percent V for the carelements. 5o bide V4C3; .03Xpercent W for the carbide W2C TABLE III Efiects of combinations of Cr, V, W, and Mo on hardness of steels after tempering Rockwell O Hardness After Cumulative Analysls Per Cent Sum Of 2 hour Temperings At- Bar Cr,V, W, Mo 0 Cr v W Mo 1150" F. 1200 F. 1250 F. 1300 I 1350 F.

It is clear from this table that steels of the first and .OGXpercent M0 for the carbide MO2C. Now, group containing the carbide forming elements I have discovered that if the composition of the between 4.6 and 6.6% in aggregate, have much steel is so balanced that the carbon content better resistance to softening above 1200 F. than thereof is approximately equal to the total carsteels of the second group containing these elements between 7.1 and 9.4% in aggregate.

All of the data given above serve to negate the generally accepted opinion that increasing bon which can combine with the elements chromium, vanadium, tungsten and molybdenum according to the carbide formulae given above, then the resistance of the steel to softening will be amounts of carbide forming alloying elements, greatly enhanced. On the other hand, if the composition is such that the carbon content is appreciably less than the total amount that can combine with the elements, Cr, V, W, and Mo, according to the above formulae, then the resist- Many of the high alloy commercial steels contain so much of the carbide forming elements that the steels are hypereutectic, that is, they contain residual carbides right up to the solidus temance to softening is relatively poor. perature of the steel. Accordingly, when the Before presenting test data in support of this steels are heated for hardening, even though they statement, consider for a moment how the forare heated at very high temperatures, all of these mula works out for a well known commercial elements are not dissolved in the austenite. Consteel, viz., 18-4-1 high speed, as compared to a sequently the martensite which forms from this steel in accordance with the presentinvention, l0 austenite when the steel is cooled to room temasset forth in the following Table IV. perature, does not contain the maximum of the TABLE IV Amounts of 0 Which Could Combine Total Analysis Percent With Elements As Carbides Amount gfig Of Carbon 00 i hie Combine Could minus 0 or V w Mo one. v.0. w.o M020 Actual .10XCr .l8XV .03XW .06 Mo Elements Carbon,

18-4-1 High Speed Steel-.." .72 4.1 1.0 17.6 0.4 .41 .18 .53 .02 1.14 +.41 Steel ofInvention .56 1.3 1.3 2.8 1.5 .13 .23 .08 .09 .53 .03

In the steels of this table, the balance of which is iron, in each instance, the compositions of the carbide forming elements of the steels are given alloying elements in the steels. Since the resistance to softenin depends on the precipitation of alloy carbides in the tempering of the first; then follows the amounts of carbon'which could combine with each of the elements in the steel according to the formulae given above; then, in the second column from the right, the total amount of carbon which could combine with the chromium, vanadium, tungsten and molybdenum in the steel is given under the heading T; and finally, in the last column at th right, the difference between this total amount of carbon that could combine with the carbide forming elements in the steel, T, and the actual amount of carbon, C, in the steel is given under the heading T-C. As shown in Table IV, for the commercial 18-4-1 high speed steel the carbon content is considerably less than the total carbon required completely to combine with the carbide forming elements present. Therefore, the value of T is appreciably higher thanthe value of C, and the difierence between the two, T-C, is plus 0.41. On the other hand in the steel of this invention, the carbide forming elements are so balanced that the value of T approximates martensite, the full advantage of the alloyin elements is not attained unless these elements had previously been dissolved in the austenite. In order to measure the optimum resistance to softening, it is desirable, therefore, to heat the steels just short of the solidus temperature, and then quench and reheat them, and test for hardness after successively higher reheating temperatures.

Such a procedure was carried out for a series of commercial steels and for steels of this invention. In order to make the comparison as fair as possible, all steels with one exception, were heated to 2350 R, which is close to the maximum temperature permissible for 18-4-1 high speed steel. The

other steel, a vanadium-molybdenum, high speed steel, was heated to 2225 F., approximately the solidus temperature of this steel. The steels were then quenched and reheated cumulatively for periods of two hours each, at temperatures ranging from 1150 to 1350 F., in fifty-degree intervals, and. tested for hardness after each reheating. The results are shown in the following Table V.

TABLE V Comparison of resistances to softening at temperatures from 1150 to 1350 F. of steels of this invention, and commercial steels [All steels except as noted were heated to 2350 F.; quenched; and then reheated as indicated.]

, Analysis, Per Cent Bar C 'Cr V 8Mo-2V High Speed. 14W-2Cr Hot Work 9W-3Cr Hot Work 4W-5Cr Hot Work. 5Cr-Mo-V Hot Work..." 5, 087 39 5Cr-W-Mo Hot Work 5, 332 .33

Rockwell C Hardness, After Cumulative 2 hr. Temperings at W Mo T T-C 1150 F. 1200 I. 1250 F. 1300 F. 1350 F.

1 Heated at 2225 F.

the value of C, the difierence between them being only minus 0.03. r l

It will be noted that the first four steels representative of this invention in this table have better resistances to softening at temperatures over 1150 F. than 18-4-1 high speed steel. This in itself is remarkable, because 18-4-1 high speed steel has long been considered an outstanding material in this respect. It is even more remarkable when consideration is given to the fact that the alloy contents of the steels of this invention are so much lower than those of the 18-4-1 high speed steel. Furthermore, steels 5805 and 5905 representative of this invention, have, despite their very low alloy contents, better resistances to softening them all the commercial steels except the much more highly alloyed high speed steels, and the 14% W hot work steel. The savings in vanadium and tungsten that are possible in the steels of this invention are of considerable strategic value to the United States of America in times of emergency.

It will be noted in Table V that all of the commercial steels have values of (T-C) which are plus 0.20, or higher; whereas the steels of this invention all have values of (T-C) less than plus 0.10. In all the steels of this invention I keep the (T-C) values under plus 0.15.

contains alloying elements in amounts greater than necessary to combine with the carbon in the ratios mentioned above, that is, when the composition is so balanced that the value of T-C is greater than about plus 0.15%, the growth of the alloy carbide particles seems to be facilitated. On the other hand, if the steel contains just enough or somewhat less than enough of the alloying elements to combine with the carbon, that is, when the composition is so balanced that the value of T-C is zero or less, the growth of the alloy carbide particles is retarded. Furthermore it appears that the more complex the composition, that is, the greater the number of the alloying elements the steel contains the more difficult it is for the carbide particles to grow, and, therefore, the more resistant to softening the steel is.

For this reason I prefer the steels of this invention to contain all of the carbide forming elements referred to previously, namely, chromium, vanadium, tungsten and molybdenum, rather than just one or two of them. The effectiveness of the larger number of these elements is shown in Table VI.

TABLE VI Efiect 0 four versus three carbide forming elements on the resistance to softening of the hardened steels Rockwell C Hardness After Cumulative Analysis, Per Cent Tempermg Bar T T-C C CI V \V IVIO Bill. 1100 F. 1150 F. 120Q F. 1250 F. 1300 1350 F.

2224 1. 1 1. 4 2. 3 1. 9 F6 58 O8 59 57. 5 54. 5 50 46. 5 42 2220 48 1. 4 1. 4 2. 9 Fe 57 09 58 55. 5 52. 5 47. 5 43. 5 39 It will be further noted in Table V, that the higher the T value in the steels of this invention the higher the hardness. For applications, where wear resistance is of prime importance I, therefore, balance the steel to have a high T value, and where toughness is the more important requirement, I keep the T value low. Generally for reasons of alloy economy and proper balance of hardness and toughness I prefer the T values to range between 0.3 and 1.00. Since the carbon contents of the steels of my invention are close to the T value, I limit the T value to a maximum of 1.00 for the purpose of maintaining good forgeability of the steels. However I do not confine myself to the use of these steels in the forged condition because I have made useful hot working tools of sand castings of the steels of this invention.

Although it is not yet altogether clear what the mechanism is that controls the resistance to softening of steels of the type being discussed, it seems reasonable to conclude that the secondary hardening when the steel is reheated at temperatures above about 900 F. is attributable to the precipitation of alloy carbides, and that the subsequent softening when the steel is heated at higher temperatures is attributable to the agglomeration and growth of these carbide particles. Since it is true that some steels soften much more rapidly with a given heating than others, it seems reasonable to assume that some alloy carbides are more resistant to growth than others, and that this resistance to particle growth, which accounts for resistance to softening, is attributable to the composition of th steel. When the steel It seems clear from this tabulation that even though the T and T-C values are practically the same for both steels, Bar 2224 containing all four alloying elements is more resistant to softening at temperatures over 1100 F., than Bar 2220 which contains only three of the alloying elements.

Since the carbon factors for the elements vary from 0.3 for tungsten to .18 for vanadium, I con sider it desirable to vary the contents of these elements in the steels of this invention so that the carbon combined with each element is at least about .03%. Thus the lower limits for these elements would be about 0.3% for chromium; 0.2% for vanadium; 1.0% for tungsten; and 0.5% for molybdenum. In addition, and in order to insure adequate hardenability of the steel, chromium should be present in amounts greater than about 0.5%.

Since the carbon content of the steel must increase according to the formula with increase of the carbide forming elements, and since the toughness of the steel diminishes with increase of carbon, I maintain the upper limits of the alloy contents so that the carbon combined with each element is generally no greater than about 0.3%. This sets the upper limits of the elements to about 3.0% for chromium; about 2.0% for vanadium; about 10% for tungsten; and about 5% for molybdenum. However, because of the cost and strategic importance of tungsten, I preferably limit the use of the latter element to about 3.0%. Because of the cost factor I also preferably limit molybdenum to about 4%.

Although no mention has been made of manganese and silicon in the steels of this invention it is to be understood that all compositions contain up to about 1.0% of manganese and up to about 2.0% of silicon, the remainder being substantially all iron. The silicon is useful for imparting better scale resistance in some of the compositions used for high temperature bolting service and for hot work forging applications.

I claim:

1. An alloy steel of relatively low alloy content which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to said steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 F., said steel containing about: 0.5 to 3% chromium, 0.2 to 2% vanadium, 1 to 3% tungsten, 0.5 to 4% molybdenum, 0.3 to 1.2% carbon, up to 3% in aggregate of other elements which do not substantially impair said resistance of the steel against softening, and the balance substantially all iron, in proportions such that the sum of of the chromium content, of the vanadium content, of the tungsten content and 1 of the molybdenum content, shall total at least 0.3% but shall not exceed 0.15% plus the carbon content.

2. An alloy steel of relatively low alloy content which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to said steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 F., said steel containing about: 0.5 to 3% chromium, 0.2 to 2% vanadium, l to 3% tungsten, 0.5 to 4% molybdenum, 0.3 to 1.2% carbon, up to 1% manganese, up to 2% silicon, and the balance substantially all iron, in proportions such that the sum of of the chromium content, of the vanadium content, of the tungsten content and g of the molybdenum content, shall total at least 0.3% but shall not exceed 0.15% plus the carbon content.

3. An alloy steel of relatively low alloy content which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to said steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 F., said steel containing about: 1 to 2.5% chromium, 0.5 to 1.5% vanadium, 1 to 3% tungsten, 0.5 to 3% molybdenum, 0.4 to 0.9% carbon, and the balance substantially all iron, in proportions such that the sum of of the chromium content, of the vanadium content, of the tungsten content and of the molybdenum content, shall total at least 0.3% but shall not exceed 0.15% plus the carbon content.

4. A forgeable alloy steel, of relatively low alloy content, which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to the steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 EX, said steel containing about: 0.5 to 3% chromium, 0.2 to 2% vanadium, 1 to 3% tungsten, 0.5 to 4% molybdenum, 0.3 to 1.2% carbon, up to 3% in aggregate of other elements which do not substantially impair said resistance of the steel against softening, and the balance substantially all iron, in proportions such that the sum of of the chromium content, of the vanadium content, Ace of the tungsten content and 3 of the molybdenum content shall fall within the range of about 0.3 to 1.0%, and shall not exceed 0.15% plus the carbon content.

5. A forgeable alloy steel, of relatively low alloy content, which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to the steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 R, said steel containing about: 0.5 to 3% chromium, 0.2 to 2% vanadium, 1 to 3% tungsten, 0.5 to 4% molybdenum, 0.3 to 12% carbon, up to 1% manganese, up to 2% silicon, and the balance substantially all iron, in proportions such that the sum of of the chromium content, W of the vanadium content, i of the tungsten content and 7100 of the molybdenum content shall fall within the range of about 0.3 to 1.0%, and shall not exceed 0.15% plus the carbon content.

6. A forgeable alloy steel, of relatively low alloy content, which is hardenable by quenching from an austenitizing temperature, and which undergoes secondary hardening on subsequent tempering at about 1100 F., and which is of a balanced composition such as to impart to the steel in the quenched and tempered condition, high resistance against softening at temperatures upwards of 1100 F., said steel containing about: 1 to 2.5% chromium, 0.5 to 1.5% vanadium, 1 to 3% tungsten, 0.5 to 3% molybdenum, 0.4 to 0.9% carbon, and the balance substantially all iron, in roportions such that the sum of of the chromium content, of the vanadium content, of the tungsten content and of the molybdenum content shall fall within the range of about 0.3 to 1.0%, and shall not exceed 0.15% plus the carbon content.

PETER PAYSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,937,334 Emmons Nov. 28, 1933 2,209,623 Houdremont et a1. July 3, 1940 Certificate of Correction Patent No. 2,565,264 August 21, 1951 PETER PAYSON It is hereby certified that error appearsin the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 7, for it read is; column 7, line 12, for them read than; column 8, line 49, for from 0.3 read from .03; column 10, line 66, list of references cited for July 3, 1940 read July 30, 1.940;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oifice.

Signed and sealed this 12th day of February, A. D. 1952.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

1. AN ALLOY STEEL OF RELATIVELY LOW ALLOY CONTENT WHICH IS HARDENABLE BY QUENCHING FROM AN AUSTENITIZING TEMPERATURE, AND WHICH UNDERGOES SECONDARY HARDENING ON SUBSEQUENT TEMPERING AT ABOUT 1100* F., AND WHICH IS OF A BLANCED COMPOSITION SUCH AS TO IMPART TO SAID STEEL IN THE QUENCHED AND TEMPERED CONDITION, HIGH RESISTANCE AGAINST SOFTENING AT TEMPERATURES UPWARDS OF 1100* F., SAID STEEL CONTAINING ABOUT: 0.5 TO 3% CHROMIUM, 0.2 TO 2% VANADIUM, 1 TO 3% TUNGSTEN, 0.5 TO 4% MOLYBDENUM, 0.3 TO 1.2% CARBON, UP TO 3% IN AGGREGATE OF OTHER ELEMENTS WHICH DO NOT SUBSTANTIALLY IMPAIR SAID RESISTANCE OF THE STEEL AGAINST SOFTENING, AND THE BALANCE SUBSTANTIALLY ALL IRON, IN PROPORTIONS SUCH THAT THE SUM OF 1/10 OF THE CHROMIUM CONTENT, 10/100 OF THE VANADIUM CONTENT, 3/100 OF THE TUNGSTEN CONTENT AND 6/100 OF THE MOLYBDENUM CONTENT, SHALL TOTAL AT LEAST 0.3% BUT SHALL NOT EXCEED 0.15% PLUS THE CARBON CONTENT. 